The final energy consumption in Hellenic residential and tertiary sector buildings represents almost one-third of the total energy budget. From this, space and water heating represents 70% of the total. Space heating and electricity generation in Hellas are based on fossil fuels, with a serious environmental impact. Despite the popularity of simple solar domestic hot water systems, active solar space heating remains, for various reasons, marginal. The aim of this paper is to demonstrate the potential of combined active space and domestic hot water solar systems, both technically and economically. An assessment of the reduction of the environmental impact is also included. The results confirm that modern solar technology constitutes a viable option for energy retrofit in the building sector in Hellas.